The invention relates generally to the reproduction of originals.
More particularly, the invention relates to a method and apparatus for the printing of colored originals.
It is known to scan a colored original along rows and columns, or point-by-point, and to generate electrical imaging signals based on the scanning operation. The signals are divided into three series respectively corresponding to the red, green and blue components of the original. The signals are processed and then converted into an optical image by means of a cathode ray tube having a screen which is coated with a phosphor. The phosphor emits radiation which is focused on negative color copy material having relatively low sensitivity in the red region of the spectrum via an objective. The copy material is successively exposed to the red, green and blue components of the image by successively moving three color filters into the path of the radiation travelling from the phosphor to the copy material.
Electronic image processing, that is, processing of the electrical imaging signals generated upon scanning an original, is being increasingly used in the reproduction of colored originals and the production of colored photographic positives from colored negatives. Here, the original is electronically scanned along rows and columns, or point-by-point, and the resulting electrical imaging signals are corrected or modified in accordance with specific criteria. The modified imaging signals are sent to a color exposure unit which converts the electrical imaging signals into an optical image for printing on photographic copy material, e.g., color negative paper. An apparatus for carrying out a procedure of this type is disclosed, for instance, in "An Experimental Cathode Ray Tube Printer", Journal of Imaging Technology, Vol. 12, No. 3, pp. 135-139. In this apparatus, the electrical imaging signals are converted into an optical image by means of a cathode ray tube having a screen coated with substances capable of emitting radiation for printing of the image. These substances include a phosphor of the type known as P 45 and yttrium sulfide doped with europium. Approximately one-quarter of the printing light is generated by the phosphor while 74% is generated by the yttrium sulfide. The resulting emission spectrum has distinct peaks of different amplitudes over the range of 380 to 720 nm. During printing, different color filters are successively moved into the path of the printing light. The filters employed in the known apparatus are gelatin filters having transmission patterns with relatively flat sides and exhibiting significant residual radiation absorption even in wavelength ranges corresponding to those colors for which the filters possess maximum transmissivity.
Due to the differing sensitivities of the copy material in the basic or primary colors, the emission pattern of the phosphor and the absorption characteristics of the filters, the amounts of printing light required in the individual colors for an original of average coloration differ significantly. In principle, it is possible to increase the brightness or intensity of an image point on the screen of the cathode ray tube by increasing the current in the cathode. However, such increase in intensity is limited because the tube may become overloaded at high light intensities and, in addition, the size of an image point increases with increasing intensity thereby reducing image sharpness. In practice, uniformly intense light emission throughout all wavelength ranges cannot be achieved with a single phosphor so that mixtures of phosphors must be created for this purpose. Aside from the sharply increased cost of a tube capable of providing this result, mixtures of phosphors have the further disadvantage that the various components age differently. Consequently, the intensities of the emissions in the different colors change in different ways during the life of the tube.
If the greatly differing amounts of printing light in the known apparatus are achieved by controlling the exposure time only, the exposure times become relatively long with an accompanying reduction in copier output.